1. Field of the Invention
The present invention relates to an image forming apparatus, particularly to an image forming apparatus provided with at least one of a copy function, a print function and a facsimile function.
2. Description of the Related Art
There is known an image forming apparatus provided with an image bearing member having an image (toner image) to be transferred to a transfer material (e.g. transfer sheet) formed on a surface thereof by using toner as developer and a transfer roller for forming a transfer nip portion by the contact with the image bearing member to transfer an image formed on the image bearing member to the transfer material in a developing device.
If this image forming apparatus forms a monochromatic image, the transfer nip portion is formed by the contact of the transfer roller with a photoconductive drum. On the other hand, if this image forming apparatus forms a color image, the transfer nip portion is formed by the contact of the transfer roller with an intermediate transfer belt. In such a state, the transfer material is conveyed to the transfer nip portion.
When the transfer material is convened to the transfer nip portion, a transfer bias is applied to the transfer roller to generate a transfer electric field between the photoconductive drum or the intermediate transfer belt (hereinafter, image bearing member) and the transfer roller. At this time, an image formed on the surface of the photoconductive drum or the intermediate transfer belt is transferred to the transfer material.
If no proper air gap is provided between the image bearing member and the transfer material, discharge occurs between the image bearing member and the transfer material, whereby a phenomenon of diffusing the toner on the surface of the image bearing member to the periphery of an original transfer position of the transfer material without being transferred to this original transfer position is likely to occur.
If the air gap is wide, the following phenomenon tends to occur. Specifically, if the air gap is wide, spot discharge is likely to occur between the transfer roller and the transfer material immediately before the transfer material reaches the transfer nip portion. Upon the occurrence of such spot discharge, the polarity of the toner to form the image on the surface of the image bearing member is locally reversed. Thus, a phenomenon (so-called white spot phenomenon) in which toner untransferred parts appear in dots occurs on the transfer material having the image transferred thereto.
Further, if the air gap is wide, discharge is likely to occur in a wide area between the transfer roller and the transfer material immediately before the transfer material reaches the transfer nip portion. such discharge has an influence over the wide area of the toner to form the image on the surface of the image bearing member. Then, the polarity of the toner is reversed in a specific area of the surface of the image bearing member. Thus, a phenomenon in which a toner untransferred part appears over a specific area occurs on the transfer material having the image transferred thereto. Other phenomena of transfer failures also tend to occur.
Thus, the following measures have been adopted to make the air gap sufficiently small. For example, a method for guiding a transfer material entering direction toward the image bearing member by a pre-transfer guide or the like is adopted. Further, a method for changing the position of the transfer nip portion to a position on the circumferential surface of the image bearing member at an upstream side in a conveying direction of the transfer material is adopted to make the transfer material having passed the transfer nip portion easily separable from the image bearing member and the transfer roller. By these measures, the air gap is made sufficiently small.
However, in such cases, if a highly rigid transfer material such as a pasteboard is used, the transfer material is likely to be bent in a thickness direction of the transfer material in a conveyance path since being conveyed while the leading end thereof is colliding with the image bearing member. If such a phenomenon occurs, a conveyance path from registration rollers disposed upstream of the transfer nip portion for feeding the transfer material to the conveyance path to the transfer nip portion is bent by being pushed by the transfer material.
If the conveyance path from the registration rollers to the transfer nip portion is bent, resistance hindering the conveyance of the transfer material increases in the conveyance path. Thus, in a conveyance process, a conveying speed of a part of the transfer material having entered the conveyance path after passing the registration rollers is slower than that of a part of the transfer material passing the registration rollers.
In such a case, there is, for example, a large tendency to occur image faults such as horizontal stripes in an image transferred to the transfer material. There is also a large tendency of not properly transferring the image due to an upward movement of the rear of the transfer material. Further, there is a large tendency of reducing an equal magnification in a conveying direction (a sub scanning direction) of an image. There are also large tendencies of other problems.